The hydrofluoro-olefin 2,3,3,3-tetrafluoropropene (HFO-1234yf, CF3CF═CH2) is a low global warming compound with zero ozone depletion potential which finds use as a refrigerant, a foam blowing agent, a monomer for polymers, and many other applications. A number of methods are known in the art for making HFO-1234yf. See, for example U.S. Pat. Nos. 8,975,454, 8,618,340, 8,058,486, and 9,061,957. See also, U.S. Patent Pub. Nos. 2009-0099396 and 2008-0058562.
Another route to HFO-1234yf is the hydrofluorination of 1,1,2,3-tetrachloro-propene (TCP), as disclosed in U.S. Pat. Nos. 8,084,653 and 8,324,436. PCT Publication No. WO 2009/003085 A1 describes the preparation of HFO-1234yf via the metathesis of hexafluoropropene (HFP) and ethylene. This process requires the use of an expensive metathesis catalyst in an organic solvent and thus not cost effective for commercial production.
These methods for making HFO-1234yf generally involve multiple steps, by-product formation, and have a low atom efficiency percentage. Atom efficiency percentage is calculated as follows:(the molecular weight of the desired product) divided by (the molecular weight of the substances formed)×100.
The thermal dimerization of fluoro-olefins has been described in the literature. See, for example, U.S. Pat. Nos. 2,427,116; 2,441,128; 2,462,345; 2,848,504; 2,982,786; and 3,996,301. See also, J. Fluorine. Chem., 2004, 125, 1519; J. Chem. Soc., Perkin I, 1973, 1773; J. Chem. Soc., Perkin I, 1983, 1064.
U.S. Pat. No. 3,996,299 describes a process for the formation of a copolymer produced from vinylidene fluoride and 2,3,3,3-tetrafluoropropylene. This process involves the cyclodimerization of a perfluoroolefin, such as perfluoropropylene, with a terminal monoolefin, such as ethylene, to produce the cyclic compound 1,1,2-trifluoro-2-trifluoromethyl-cyclobutane (TFMCB). The cyclic compound such as TFMCB is then subjected to a thermal cracking operation to produce a mixture of acyclic fluorine-containing olefins, such as vinylidene fluoride and 2,3,3,3-tetrafluoro-propylene, which can be used as monomers and/or comonomers in polymerization reactions.
The '299 patent discloses the cyclodimerization reaction can occur over a very wide range of reaction conditions. For example, the patent indicates that the reaction temperature can be in the range of 200° to 600° C., preferably 300° to 400° C., and that the reaction time in the range of about 4 to about 1000 hours, preferably 10 to 100 hours. The'299 patent also indicates that the ratio of the monoolefin to the perfluoroolefin usually is in the range of 0.1:1 to about 100:1 preferably 1:1 to about 10:1.
The '299 patent discloses that the thermal cracking of the cyclic compound at temperatures in the range of 500° to 1000° C. and preferably in the range of 600° to 700° C. It is stated that the cracking reaction can be carried out continuously by passage through a heated reactor tube maintaining a contact time in the range of 0.01 to 10 seconds.
Applicants have come to recognize several problems and disadvantages associated with the formation of HFO-1234yf according to a process as described in the '299 patent. One such problem is that the '299 patent fails to recognize the potential problem in the cracking reaction associated with olefin oligomerization at high temperatures. Other problems are the presence of HFP and ethylene (the starting material) in the cracking products along with other side products, which are not mentioned in the '299 patent. Applicants have come to appreciate that these problems would be exacerbated under many of the dimerization reaction conditions specified in the '299 patent. The final reaction product is thus a complex mixture under the specified reaction conditions, especially with large excess of ethylene to HFP ratios. Another problem is that many of the permitted ratios of perfluorolefin, such as HFP, to the monoolefin, such as ethylene, can produce undesirable reaction product results, including unwanted or detrimental by-products and/or poor conversions and/or selectivities. Similar disadvantages associated with unwanted or detrimental by-products and/or poor conversions and/or selectivities are possible within the range of reaction conditions for the cracking reaction.
At least in part as a result of the recognition of these problems with the prior art, applicants have developed new and greatly improved processes that provide significant and unexpected advantages in the production of HFO-1234yf and mixtures of HFO-1234yf and vinylidene fluoride (VDF).